Fertilization - Human Reproduction - Class 12 PDF Download

DEFINITION
The fusion of a haploid male gamete (sperm) and a haploid female gamete (ovum) to form a diploid zygote is called fertilization.

SITE OF FERTILIZATION
In human beings, fertilization takes place mostly in the ampulla of the oviduct (Fallopian tube).

• Arrival of Sperms: Male discharges semen into the female’s vagina close to the cervix during coitus (copulation). This is called insemination. A single ejaculation of semen may contain 300 million sperms.
• Capacitation: Reactivation of sperm called capacitation. It is produced in vagina. After ejaculation of sperm in vagina, cervical mucous secretory fluid dissolves inlibitory substances related to sperm & provide more energy to sperm.
  Fig: Fertilization: Generalized Acrosomal Process
• Cortical Reaction: Just after the fusion of sperm and plasma membranes of oocyte, the secondary oocyte shows a cortical reaction. The cortical granules are present beneath the plasma membrane of the secondary oocyte. These granules fuse with the plasma membrane of the oocyte and release their contents including cortical enzymes between the plasma membrane and the zona pellucida.
• Fertilization: Antifertilizin Interaction. According to this theory egg (ovum) secretes a chemical named fertilizin (composed of glycoprotein = monosaccharides + amino acids) and sperm has on its surface a protein substance called antifertilizin (composed of acidic amino acids). The fertilizin of an egg interacts with the antifertilizin of a sperm of the same species.
• Sperm Entry: At the point of contact with the sperms, the secondary oocyte forms a projection termed the cone of reception or fertilization cone which receives the sperm. The distal centriole of the sperm divides and forms two centrioles to generate the mitotic spindle formation for cell division.
• Karyogamy (Amphimixis): Sperm entry stimulates the secondary oocyte to complete the suspended second meiotic division. This produces a haploid mature ovum and a second polar body. The head of the sperm which contains the nucleus separates from the middle piece and the tail and becomes the male pronucleus.
  The second polar body and the sperm tail degenerate. The nucleus of the ovum is now called, the female pronucleus. The male and female pronuclei move towards each other. Their nuclear membranes disintegrate. Mixing up of the chromosomes of a sperm and an ovum is known as karyogamy or amphimixis. The fertilized ovum (egg) is now called zygote.

SIGNIFICANCE OF FERTILIZATION
(i) Activation of egg to start cell division and embryonic development.
(ii) Restoration of the diploidy and recombination of two different sets of genes.

Semen

• Milky, viscous, alkaline fluid.
• pH = 7.3
• Quantity is 2.5 – 4 ml at one ejaculation

Maximum growth in human embryo occurs in fourth month of pregnancy and minimum in last months.
Pseudopregnancy = Due to hormonal disturbance, false symptoms of pregnancy may appear.

Menarche – Beginning of menstrual cycle in females.

• Size of human ovary – Length × Width × Thickness 3 to 3.5 cm 2 to 2.5 cm 1 to 1.8 cm
• Diameter of Graafian follicle – 1 to 1.5 cm.
• Fallopian tube length – 10 cm to 12 cm.

(i) Hermaphroditism: Those animals who have both type of sex organs are called Hermaphrodite animals, it is found in tapeworms and earthworms.

(ii) Capacitation: Reactivation of sperm called capacitation. It is produced in vagina. After ejaculation of sperm in vagina, cervical mucous secretory fluid dissolves inlibitory substances related to sperm & provide more energy to sperm.

(iii) Precautious puberty: Menstruation occurs before the age of 9 years. Pregnancy is detected with the help of HCG in urine of pregnant lady.

(iv) Cryptomenorrhoea: Occurrence of menstrual symptoms without external bleeding.

(v) Dysmenorrhoea: Painful menstruation is called Dysmenorrhoea.

(vi) Menorrhagia: A normal menstrual blood loss is 50-80 ml, and does not exceed 100 ml. In menorrhagia the menstrual cycle is unaltered but the duration and quantity of the menstrual blood loss are increased.

(vii) Polymenorrhoea: In polymenorrhoea or epimenorrhoea, the menstrual cycle is reduced from the normal of twenty-eight days to a cycle of two to three weeks and remains constant at that increased frequency.

(viii) Male hormonal contraceptive: Research is on for development of a male contraceptive by administration of progesterone hormone.

(ix) Oral contraceptive pill: It is commonly used method for contraception. It is in the form of oral tablet. If menstrual cycle is to start on Ist day then OCP is taken from 5th day & regulary up to 21st day. Remaining 7 day Iron & folic acid tablet is given instead of hormonal preparations.

Mechanism of Action
(i) It inhibits Ovulation (main action)
(ii) It prevents fertilization due to atrophy or hypertrophy of endometrium.
(iii) Formation of thick cervical mucus plug, so that entry of sperm is restricted.